The MGB was powered throughout its production life by the BMC 1798cc B-series four-cylinder engine, a development of the unit originally designed for the MGA. The engine capacity was increased to 1798cc for the MGB, and blocks can be identified by the legend ‘1800’ cast into the left-hand side of the crankcase.
Engine Identification
Engine specification changed numerous times between 1962 and 1980, with each significant revision marked by a new engine number prefix. The prefix is stamped on a plate on the right-hand side of the block, just below the cylinder head, and is the single most important reference when selecting parts.
From May 1962 to August 1971, engines in the 18G through to 18GK series were painted maroon red. From August 1971, the 18V-series engines were painted black, and this colour continued to the end of production.
Production Development
The principal milestones in the engine’s development are as follows. The original 18G engine of 1962 featured a three main bearing crankshaft, open crankcase breathing, and dynamo charging. The 18GA introduced closed circuit breathing in early 1964 but retained the three-bearing crankshaft. In October 1964, the landmark 18GB series brought a five main bearing crankshaft, a substantial redesign that significantly improved bottom-end strength and refinement, and which would remain the foundation of all subsequent MGB engines. The 18GD of 1967 introduced the alternator and the fully synchronised four-speed gearbox.
The 18GG followed in 1968 with carburettor crankcase ventilation in place of the earlier PCV valve system. From 1971, the 18V-series engines brought a revised cylinder head with larger inlet valves, single valve springs, and a succession of further changes across chrome bumper and rubber bumper production, most notably the reversion to smaller inlet valves and the introduction of the offset rocker pedestal oil feed on rubber bumper engines from September 1974.
Parts Compatibility
These changes have a direct effect on parts compatibility. The engine prefix determines which cylinder head, water pump, inlet and exhaust manifolds, carburettors, rocker assembly, timing chain type, oil filter arrangement, crankcase breathing components, and many other items are correct for the engine fitted. A full engine prefix reference table is available as a downloadable guide, summarising the key identification features and parts differences across all MGB engine series.
Replacement & Non-Original Engines
Because many MGBs are now over sixty years old, it is common to find engines that have been replaced with a different specification unit at some point in the car’s life. Factory reconditioned replacement engines were supplied under the Gold Seal label and can be identified by their distinctive gold paint finish and special engine number prefixes beginning with 48G or BHM. More recently, remanufactured engines from specialist suppliers carry their own serial number series. In all cases, checking the engine number prefix, rather than assuming the engine matches the car’s original specification, is essential to ensure the correct parts are selected.
The MGOC Spares range encompasses standard reconditioned and performance engines, Stage II cylinder heads, camshaft kits, rocker assemblies, gaskets, oil filtration, cooling system components, and the full suite of individual engine spares to support rebuilds at every level. If you are unsure which engine is fitted to your car, or you need help identifying the correct parts for your specification, our technical team is available to assist.